1. Field of the Invention
The present invention relates to an improved process for the preparation of aniline by catalytic hydrogenation of nitrobenzene in the gas phase.
Aniline is known to the expert as an important intermediate product, for example for the preparation of dyestuffs and polyurethanes.
2. Description of the Related Art
Palladium-containing supported catalysts are known for the hydrogenation of nitrobenzene and other nitroaromatics. In addition to the catalytically active metal, the materials employed as the catalyst support are also decisive for the activity of a catalyst. Because of their surface nature, these support materials can lead to the formation of undesirable by-products. Aluminium oxides which have been converted completely or partly into spinel are known as supports from DE-OS (German Published Specification) 2,135,155 and DE-OS (German Published Specification) 2,244,401. Further processes for the hydrogenation of nitrobenzene are known from DE-OS (German Published Specification) 2,848,978 and DE-OS (German Published Specification) 2,849,002.
The catalysts known to date for the hydrogenation of nitroaromatics in the gas phase show disadvantages which limit their use in the chemical industry. Thus, although the initial yield in the hydrogenation of nitrobenzene to aniline is quite high at 99%, the known catalysts rapidly lose their activity, for example by poisoning or by the deposition of tar-like precipitates, so that only short service lives result. In this case,, the hydrogenation must be interrupted in order to regenerate the catalyst. The frequent interruptions and the expensive nature of the regeneration are uneconomical for large-scale industrial plants. A further disadvantage of the catalysts known to date is their irregular activity, which frequently manifests itself in an over-reactivity to give undesirable by-products. If larger concentrations of noble metal or diverse combinations of different elements are used to increase the service life, the high noble metal requirement has the effect of an undesirable cost-causing agent in the industrial realisation of such processes (DE-OS (German Published Specification) 2,849,002). Such catalysts prepared from many elements are moreover more cumbersome to prepare and to dispose of.